1. Field of the Invention
This invention relates to power supplies, and more particularly relates to improving efficiency in a switching power supply operating near zero load conditions.
2. Description of the Related Art
A power supply, sometimes known as a power supply unit or PSU, is a device or system that supplies electrical or other types of energy to an output load or group of loads. A power supply, in some embodiments may be configured to convert power in one form to another form, such as converting AC power to DC power. The regulation of power supplies is typically done by incorporating circuitry to tightly control the output voltage or current of the power supply to a specific value. The specific value is closely maintained despite variations in the load presented to the power supply's output, or any reasonable voltage variation at the power supply's input. A switching power supply is a power supply that incorporates a switching regulator that typically switches a power transistor, usually a metal-oxide-semiconductor field-effect transistor (“MOSFET”), between saturation and cutoff with a controller to maintain a constant output voltage.
For example, in an electrical device such as a computer, a switching power supply is typically designed to convert an AC voltage input such as is traditionally provided by a conventional wall socket, into several low-voltage DC power outputs for transmission to the internal components of the device. Conversion is sometimes performed in stages that may include different power supply topologies such as a rectification stage, a boost stage, buck stage, or other topology.
During light load conditions and zero load conditions in a switching power supply, almost all losses are the result of interelectrode capacitances of the MOSFETs used as switches in the circuitry of the power supply. These losses are sometimes referred to as switching losses. Particularly in a power supply designed for high power applications, the MOSFET capacitances can be so large that the power supply may require a shutdown during low load periods due to excessive power dissipation. For example, in a typical switching power supply, the power loss during a no-load condition may be as high as 45-50 watts, and this power loss continues to increase as the MOSFET temperature rises. In cases where there is little or no forced air flow, the MOSFETs can reach very high temperatures which may result in catastrophic failure of the power supply. Conventional power supplies have failed to adequately address the problem of high switching losses in a switching power supply operating in a low load condition.